Retroviral entry into cells depends on envelope glycoproteins, whereby receptor binding to the SU subunit triggers membrane fusion by the transmembrane (TM) subunit. The human T cell leukemia virus type I (HTLV-1) is a retrovirus with a wide geographic distribution, associated with adult T cell leukemia and tropical spastic paraparesis/HTLV- 1 -associated myelopathy. We have crystallized the ectodomain of gp2 1, the TM from (HTLV- 1), as a maltose-binding protein (MBP) chimera. No diffraction data have yet been collected with synchrotron radiation using the optimized crystals. A high resolution monochromatic cryo data set is required for drug design efforts using the structure of gp2l. The structure in this crystal form will be of interest to assess conformational differences that may shed light on the process of membrane fusion, and the association of MBP and gp2l.